FRONT DERAILLEUR BASICS

FRONT DERAILLEUR BASICS
Figure 1 - A standard front derailleur
The front derailleur on a bicycle has the task of “derailing” the top side of the chain entering the front
chain ring so that it can be moved from one chain ring to the other. The front derailleur also has the task
of keeping the chain on the chain ring that it is currently wrapped around, essentially working as a chain
guide so that the chain does not derail unintentionally while travelling over harsh terrain.
The front derailleur will only operate properly when the chain rings are engaged in the clockwise
rotation since it is the top side of the chain (drive side) that must be forced onto the larger or smaller
chain ring in order to change the gear ratio. Also, the chain used for a multi-speed bicycle is thinner pitch
than the type used on a single speed cycle, and it is also more flexible from side-to-side, allowing it to be
forced over to the next chain ring. Adjustment and setup of a front derailleur is a simple task once you
understand the basic operating principals.
The typical front derailleur shown in Figure 1 represents the most commonly available steel plate style
that most modern cycles are equipped with. The front derailleur sits just over the largest front chain
ring, held in place by the clamp that is secure around the seat tube. When the shifting lever is moved, the
chain cage is pushed or pulled, forcing the chain to jump off the current chain ring and onto the next one.
Newer shifting hardware is “indexed”, meaning that the
shifting hardware is matched to the derailleur so that one
“click” translates into one shift. Older shifters are simple
levers that allow you to move the derailleur position in
either direction until you have shifted gears. The older
systems are much easier to setup and maintain, but do
make an annoying ratcheting sound as you move between
gears.
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Front Derailleur
Figure 2 - The rear derailleur takes up chain slack
To make it possible for the chain to ride on multiple diameter chain rings, a chain that is long enough to
wrap the two largest rings must be used. This requires that the “slack” or return chain be picked up and
pulled somewhat tight by the spring loaded rear derailleur. Figure 2 shows the rear derailleur pulling
back on the lower chain, keeping the tension tight. The lower part of a bicycle chain is called the “return
chain” because it never has any real tension, nor does it drive the transmission in any way.
Figure 3 - The cage derails the chain
Figure 3 shows the movement that the front derailleur makes as it changes gears. The steel cage pushes
against the side of the chain, causing it to jump over to the next chain ring where it is picked up by the
sprocket teeth and wrapped around that chain ring in the clockwise (pedaling) direction. Back pedaling
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Front Derailleur
while you shift will jam up the chain since the front derailleur cannot force the chain off of the chain ring
in the counter clockwise rotation.
Figure 4 - The high and low limiting screws
High and low limiting screws (shown in Figure 4) must be properly adjusted to allow the full range of
motion without having the chain become shifted right off of the last chain ring.
Adjustment of the min and max setting is accomplished by turning the adjuster screw with a Phillips
screwdriver as shown in Figure 4 until the shifting range of motion is set properly. Having the cycle up
on a stand so that you can turn the cranks while making the adjustment is the best way to set the min
and max screws properly.
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Front Derailleur
Figure 5 - The shifter cable clamp
The shifting cable shown in Figure 5 is responsible for pulling the derailleur body in one direction
against the built in spring that will push it back in the other direction as the cable relaxes. Some older
indexed shifting systems use a dual push and pull cable setup, but these are fairly rare because they are
being a real pain to setup and maintain.
Also shown in Figure 5 is the small bolt and nut that secure the end of the cable to the derailleur arm.
This bolt may have a small hole to allow the cable to be “threaded” into it, or it may simply squash the
end of the cable by clamping down on it.
Figure 6 - Removing the derailleur
To remove a front derailleur from a bicycle, loosen the nut or bolt as shown in Figure 6 so that the clamp
can be pulled away from the seat tube. The hinged clamp will open up wide enough to remove the
derailleur from the tubing once the nut and bolt have been taken out of the slot. There may also be a
small nut and bolt at the end of the cage that will allow the chain to be removed from between the plates,
but not all derailleurs have this option. If your derailleur does not have a removable cage bolt, then you
will need to open the chain using a chain link tool to completely remove the derailleur.
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Front Derailleur
Figure 7 - The cage is usually parallel to the chain
When you are creating a new cycle from scratch, you will often need to plan for both a front and rear
derailleur. On a mono-boom frame such as a trike, there will be no seat tube, so some provisions need to
be made to allow the front derailleur to be installed on the proper place over top the front chain ring.
Our Atomic Zombie plans use an adjustable bottom bracket with an integrated front derailleur tube as
shown in Figure 7 (the Warrior Trike), so that the front derailleur is automatically placed in the correct
position when adjusting the distance between the seat and cranks for riders of varying leg lengths. Also
note that the orientation of a front derailleur is typically set at 90 degrees to the drive chain line as
indicated by the black line.
Figure 8 - Front derailleur on the Marauder Lowracer
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Front Derailleur
Figure 8 shows another mono boom frame (the Marauder), using an integral front derailleur tube and
sliding bottom bracket that allow the distance between the cranks and set to be adjusted while keeping
the angle of the front derailleur correctly set. Now you know how the front derailleur works. Be sure to
read the Rear Derailleur Basics tutorial, too.
Free bike building tutorials:
Using a chain link tool
Basic fishmouth cutting
Arc welding
Cranks and pedals
Front derailleur
Cables
Coaster brakes
Free bike projects:
Kids electric trike
Choppers
Mountain bike tandem
Bicycle bearing basics
Lacing wheels
Frame chopping
Bike chains
Rear derailleur
Freewheel
Head tubes
Sidewinder stunt bike
Velomobile
World Record SkyCycle
Head tube bearings
Salvaging wheel parts
Bicycle autopsy
Rake and trail
Brakes
Gooseneck
Detachable tandem
Tall bikes
Spin Scooter
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Simple SWB recumbent
and more!
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